Where to start? Designing a heating system for our retrofit/extension

[lookseehear]

Hi all, hoping for a nudge in the right direction. Our house was two 1800ish miners' cottages which were knocked together with limestone/rubble filled 500mm thick walls. It was extended in the 1970s to the rear with a block cavity wall, albeit only a 50mm cavity, and faced with limestone. The roof has poor/negligible insulation currently and the floors are mostly solid/uninsulated concrete. We put in a worcester bosch external oil boiler with an unvented cylinder about 3 years ago because the old boiler was 30+ years old and we couldn't keep it going any longer. The windows are mostly blown aluminium DGUs which are draughty, and in general it's a draughty house. When we put in the new boiler I also put in a Wiser heating control system with smart TRVs which has helped keep heating bills down because we only heat the rooms we need when we need them, but in general it's a cold house.

 

We've just started an extensive extension/renovation which will result in some big changes.

• We're going 'upside down' with bedrooms downstairs and living space upstairs to take advantage of better views increase ceiling height and natural light with vaulted ceilings/roof lights and allow us to open up the living space more easily
• We'll replace all windows with new TGUs and all doors will be replaced
• We're replacing the roof, so will insulate to building regs (0.15 U value)
• We're having a new ground floor extension which will have our bedroom in, which we will put wet underfloor heating in as we'll be insulating floor, walls and ceiling to modern standards
• We're replacing kitchen and bathrooms

 

I'm in a bit of a bind about what to do with the heating at the moment. My ideal solution would be an ASHP driving wet UFH throughout the downstairs, but this means we would need to dig out and replace the floors to add insulation. I'd like to use an air to air heat pump upstairs to heat the open plan living space, and also provide cooling in the summer.

 

If we decide not to dig up the floors, that means putting rads in the other rooms downstairs or using an overlay wet UFH heating system, but if we're keeping radiators, maybe we also keep the boiler given it's only a few years old.

 

I know that the improvements we're making will make a huge difference to the heating performance in the house, but I'm still unsure what the best option is, and what the second best might be if the best option proves too costly. Based on the estimates on the heatgeek website, a heat requirement of 50W/m2 would seem reasonable, which would mean we need roughly 5.5kw for downstairs and 4.5kw upstairs.

 

Probably some kind of heat loss calculation would be helpful, but I'm not really sure where to go for this, or how accurate it would be given the renovations are evolving and not set in stone. Should we just keep it simple and stick with rads everywhere but in the new extension which is fully insulated? I'd rather not do this if possible, as we'd like to move to an ASHP and get rid of the boiler, and like the idea of UFH in bedrooms and bathrooms.

 

Another option is to insulate the solid stone walls, and we had looked at EWI but decided against it for now due to the cost, change of appearance and apparent risks involved with getting it right.

 

Literally any thoughts and opinions are valid here, other than about the layout, which we're very set on at this point.

[Conor]

Controversial suggestion, but you could save the £15k+ you'd need for a new heating system and spend it on insulating the walls and keep the oil boiler.  Spending more on insulation will make more of a difference than anything else. UFH is more costly to run than rads, and is a no goer unless you've a decent amount of insulation in the floor (150mm+).Do you have the room inside for internal wall insulation?

[ReedRichards]

On 02/10/2024 at 06:54, Conor said:

UFH is more costly to run than rads

 Not wet UFH, surely?

[Conor]

28 minutes ago, ReedRichards said:

 Not wet UFH, surely?

Yes, been covered here before. You're directly heating the ground, essentially. Applies only to ground floor of course.

[ReedRichards]

Surely it depends on how much insulation you have under your slab?  And you can probably run your UFH at a lower temperature than radiators which should reduce the running cost?  Otherwise why on earth does anyone go to all trouble of installing UFH and the risk of having to dig it up later should anything go wrong?

[lookseehear]

On 02/10/2024 at 06:54, Conor said:

Controversial suggestion, but you could save the £15k+ you'd need for a new heating system and spend it on insulating the walls and keep the oil boiler.  Spending more on insulation will make more of a difference than anything else. UFH is more costly to run than rads, and is a no goer unless you've a decent amount of insulation in the floor (150mm+).Do you have the room inside for I ternal wall insulation?

You've got me thinking about internal insulation again, but given that the walls of the old part of the house are solid/rubble filled limestone I'm a little nervous due to condensation risks. It would also change the thermal mass of the building somewhat, as we'd only be heating internal air and furniture etc vs now having some thermal mass in the walls. I wouldn't try to get the best U value from these walls, as I understand going below 0.3 is where the highest risks are for interstitial condensation, so improving to something like 0.4-0.6 might be sensible with some breathable internal insulation which also wouldn't take up too much floor space.

 

The overwhelming reason for wanting UFH though is because we currently wear slippers from September to April/May due to cold floors, and for both my wife and I it would be a huge improvement to thermal comfort to have warm feet, particularly in bathrooms, but bedrooms would be lovely too. Although quite a lot of heat would be lost through the floor, warming the bedrooms in the morning would mean that heat would move upwards through the house into the living space during the day, which reduces the heat requirement of the living space fairly significantly (I would imagine).

[lookseehear]

20 minutes ago, ReedRichards said:

Surely it depends on how much insulation you have under your slab?  And you can probably run your UFH at a lower temperature than radiators which should reduce the running cost?  Otherwise why on earth does anyone go to all trouble of installing UFH and the risk of having to dig it up later should anything go wrong?

 

It seems that anecdotally people with low profile UFH heating systems with not much insulation still really like them, despite the idea that they increase heating costs. Maybe the backlash will come in a year or two as it feels like low profile UFH is still relatively new to market.

 

I've seen calculations referenced multiple times on BH that even with 300mm of EPS, 10% of heat from UFH is lost through the floor. This seems high to me and (I believe) is based on the assumption that the ground is always at 8 degrees C, which it will be if you go low enough, but I suspect is not the case directly under a floor slab. These things are complex and trying to navigate a path between the science and the experience of people who live with the systems is difficult without going insane!

[Iceverge]

1 hour ago, ReedRichards said:

Surely it depends on how much insulation you have under your slab?  And you can probably run your UFH at a lower temperature than radiators which should reduce the running cost?  Otherwise why on earth does anyone go to all trouble of installing UFH and the risk of having to dig it up later should anything go wrong?

 

It's fashionable.

It's cheap to do on a new build.

You don't have to juggle furniture with radiators.

People love the feeling of a hot floor,however.......

 

Hot floors are a sign of very inefficient UFH. An efficient UFH system with a huge chunk of insulation underneath will feel the same as a non heated floor with a huge chunk of insulation underneath. Not hot - Not cold. By all means stick some electric UFH just under the bathroom tiles for a little bit of luxury in the morning post shower. 

 

My 2p worth, install big pipes and rads and continue with the boiler. It'll run more efficiently at low flow temps anyway I think . Calling @JohnMo? 

 

When it dies swap for an ASHP.

 

Get some insulation on the walls, anything is worth it. Parged brick, battened service cavity with mineral wool and then plasterboard my favourite method at the moment. 

 

Get your airtighess strategy nailed..

 

Install some kind of continuous mechanical ventilation. Even a few dDCV fans will make a world of difference to the health of the occupants and the building. 

 

Good luck and welcome to the forum. 

 

 

 

Edited Thursday at 10:48 by Iceverge

[Iceverge]

1 hour ago, lookseehear said:

You've got me thinking about internal insulation again, but given that the walls of the old part of the house are solid/rubble filled limestone I'm a little nervous due to condensation risks. It would also change the thermal mass of the building somewhat, as we'd only be heating internal air and furniture etc vs now having some thermal mass in the walls. I wouldn't try to get the best U value from these walls, as I understand going below 0.3 is where the highest risks are for interstitial condensation, so improving to something like 0.4-0.6 might be sensible with some breathable internal insulation which also wouldn't take up too much floor space.

 

The overwhelming reason for wanting UFH though is because we currently wear slippers from September to April/May due to cold floors, and for both my wife and I it would be a huge improvement to thermal comfort to have warm feet, particularly in bathrooms, but bedrooms would be lovely too. Although quite a lot of heat would be lost through the floor, warming the bedrooms in the morning would mean that heat would move upwards through the house into the living space during the day, which reduces the heat requirement of the living space fairly significantly (I would imagine).

 

I think I'm going to start a therapy group for confused DIYers regarding thermal comfort, energy usage and building health. 

 

Don't worry, I too was once afflicted. 

 

Symptoms include using the phrases 

 

"Thermal Mass"

"Breathable/Breathability"

"My old builder/architect said"

"ASHP/Gas/Solar is always or never either excellent or terrible"

"It's Eco isn't it!"

 

Confusing and conflating Airtightness and breathability (whatever that is) and ventilation. 

 

Mixing up environmental pollution, climate change, embodied carbon, recyclability running costs and things that are green and indeed greenwashed.

 

 

Don't worry, you are not alone. 

 

 

 

 

 

 

[Mike]

On 02/10/2024 at 07:54, Conor said:

you could save the £15k+ you'd need for a new heating system and spend it on insulating the walls and keep the oil boiler.  Spending more on insulation will make more of a difference than anything else. UFH is more costly to run than rads, and is a no goer unless you've a decent amount of insulation in the floor (150mm+).

+1. Take a look again at the EWI & IWI options.

 

59 minutes ago, Iceverge said:

Get your airtighess strategy nailed

Install some kind of continuous mechanical ventilation

+1 again.

 

If you still have spare cash after all the above, then consider UFCH, with plenty of insulation below the slab.

[Nickfromwales]

If introducing a single zone of UFH to an existing oil boiler, you may also run in to issues with short cycling. This would require either certain rads coming on in unison as heat loss (hall / stairs / bathrooms), or a buffer tank being added to the UFH circuit, or the UVC being swapped out to a thermal store (with DHW coil inside it for hot water).

 

You'll also need independent (additional) heating controls for the UFH'd areas to 'zone' them; these will need set-back times that start sooner / end sooner than the areas with rads.

 

Ahhhh....the joys....

 

UFH here would be a 'want' vs a 'need', so forget any savings or economics tbh. You either do an overlay system and suck up the inefficiencies or dig out and fit lots of insulation, either option will have cost and performance impacts / pros / cons / caveats to swing each argument for / against, so choose wisely.

 

UFH well installed is the best thing of all, but poorly installed (then poorly performing) UFH is utter garbage to live with. Dealing with draughtproofing will be your No.1 priority in any of these cases, to lower the amount of heat required in the first place, so always consider a fabric-first approach at all times.

 

Further note that the closer you get to 'airtight' the closer you get to needing to fit mechanical (forced) ventilation too, so yet another can of worms to then consider for Part O....!

[ReedRichards]

8 hours ago, lookseehear said:

I've seen calculations referenced multiple times on BH that even with 300mm of EPS, 10% of heat from UFH is lost through the floor. This seems high to me and (I believe) is based on the assumption that the ground is always at 8 degrees C, which it will be if you go low enough, but I suspect is not the case directly under a floor slab.

 

No, it will be hotter because you are heating it with your UFH.  Or even if not you will still be losing heat through the floor but if it's 10% with UFH it must be about 5% without; given the same floor covering.

 

I suppose the worst case would be if the heat that penetrates the insulation can escape laterally to something that is colder than 8 degrees C.  So you would have to level the ground, cover that with an appropriate membrane then insulation but around the edge of that insulation you need to dig a trench down to where the ground really is 8 C and fill that trench with insulation.

[SteamyTea]

On 03/10/2024 at 12:23, Iceverge said:

I think I'm going to start a therapy group

Do it.

Be fun to see how sustainable it is.

[SteamyTea]

On 03/10/2024 at 11:11, lookseehear said:

I've seen calculations referenced multiple times on BH that even with 300mm of EPS, 10% of heat from UFH is lost through the floor. This seems high to me and (I believe) is based on the assumption that the ground is always at 8 degrees C

Not going to be far out as you only need to heat at the coldest, and generally wettest, times of the year.

 

As for going with radiators, how much area can you fit, can you easily fit larger radiators (maybe a total of 60% of the floor surface area as you don't totally pipe out all the floor area).

How about plinth/fancoil units, they can give a quick boost and some are pretty quiet.

 

Easy and cheap solution is below, it is only Canadians that make it law to take your shoes off indoors.

 

 

[lookseehear]

9 hours ago, SteamyTea said:

Not going to be far out as you only need to heat at the coldest, and generally wettest, times of the year.

 

As for going with radiators, how much area can you fit, can you easily fit larger radiators (maybe a total of 60% of the floor surface area as you don't totally pipe out all the floor area).

How about plinth/fancoil units, they can give a quick boost and some are pretty quiet.

 

Easy and cheap solution is below, it is only Canadians that make it law to take your shoes off indoors.

 

 

 

Good point about time of year - you're probably right.

 

We haven't thought about where radiators would go, but that was part of the reason for wanting UFH, so that we didn't have that to think about. I'd need to go back to the plans and see where the best place would be in each room. One issue we have is that we are on heating oil, so we are more restricted vs even gas where you can lower flow temperatures to improve efficiency. I imagine that we might need some kind of thermal store/buffer to run the UFH in the main bedroom (which will be fully insulated) on oil, which means it isn't just reusing the boiler and hot water tank and sticking in some new radiators.

 

I've seen some of the discussion about fancoil units but I assume they would also require a ASHP or thermal store? I like the idea of air heating because of how quickly you can change the temperature, which is why I was leaning towards having an air to air unit in the open plan living space.

 

On 03/10/2024 at 11:46, Iceverge said:

Get some insulation on the walls, anything is worth it. Parged brick, battened service cavity with mineral wool and then plasterboard my favourite method at the moment. 

Is this method suitable for solid stone/rubble filled walls (sorry but I have to use the word 'breathable')? Is there more detail on what you're proposing?

 

On 03/10/2024 at 11:46, Iceverge said:

My 2p worth, install big pipes and rads and continue with the boiler. It'll run more efficiently at low flow temps anyway I think . Calling @JohnMo? 

 

When it dies swap for an ASHP.

 

I'm not sure this is the case with oil boilers. The installer of ours said that downregulating the burner too much isn't good for it. Oil boiler tech seems a bit behind gas boiler tech in terms of regulating the power output.

 

On 03/10/2024 at 13:25, Nickfromwales said:

If introducing a single zone of UFH to an existing oil boiler, you may also run in to issues with short cycling. This would require either certain rads coming on in unison as heat loss (hall / stairs / bathrooms), or a buffer tank being added to the UFH circuit, or the UVC being swapped out to a thermal store (with DHW coil inside it for hot water).

 

I like the idea of a thermal store to use excess solar for example (imagine guilt free long showers 😆) but I've been following the HeatGeek YouTube channel and they seem to believe that to have a really efficient ASHP system you need to have no buffer tank, just run weather compensation with no zoning, keeping the lowest flow temps with the highest uptime possible, so I don't want to buy into a system that if we do upgrade to ASHP in the future becomes redundant.

 

You've got me fairly convinced on the idea of investing in insulation rather than UFH, however we still won't be putting in EWI for a variety of reasons, so I think we'll be putting EPS bead CWI in the existing extension (50mm cavity between blockwork) and I need to work out what we're going to do with the solid limestone walls. If we can avoid lime plaster that would work well with our builder, who says he 'likes to throw a handful of cement in just to make it go off', so I need to find a system that

1. Doesn't wreck the 'breathability' of the walls
2. Doesn't encroach too much into the room (the walls are already 500mm+ downstairs so I'd want to avoid making them significantly thicker)
3. Ideally uses conventional plasterboard
4. Can be worked into an airtightness strategy

(answers on a postcard)

 

I'm also nervous about IWI because of potential cold bridges in the void between ground floor ceiling and first floor floorboards. So much to learn and investigate and so many potential pitfalls!

[Jilly]

I made a post called ‘Interrelated Concept Confusion’ when I was at your stage. You are doing the right thing making sure you are completely up to speed on the pros and cons (and be aware of the head explosion it will cause via sleepless nights) so you can stand your ground with your builder on the compromises/decisions you have made, and check back here when they throw up objections. Take your time. 
 

 

 

[JohnMo]

On 01/10/2024 at 23:29, lookseehear said:

UFH throughout the downstairs

Not sure that would be the best solution, the floor takes an age to heat up then an age to cool down. So chasing your tail to get the right temp for sleeping.

 

Upstairs if going A2W no need for A2A, just use a fan coil connected to A2W HP. Then you have heat and cool in a single unit.

On 01/10/2024 at 23:29, lookseehear said:

Probably some kind of heat loss calculation would be helpful

Yep

 

If you keep oil, consider a thermal store, to provide DHW and a buffer for the oil boiler to give a decent run time, especially if you have zoned heating. Pul the CH water direct from buffer.

On 01/10/2024 at 23:29, lookseehear said:

When we put in the new boiler I also put in a Wiser heating control system with smart TRVs which has helped keep heating bills down because we only heat the rooms we need when we need them, but in general it's a cold house.

This would be rubbish with a heat pump. You will also need to be careful with the oil boiler short cycling, with lots of zones

 

On 03/10/2024 at 11:05, lookseehear said:

overwhelming reason for wanting UFH though is because we currently wear slippers from September to April/May due to cold floors

Good UFH doesn't equal a warm or hot floor, poorly installed does. Well insulated, you are flowing mid to high 20s, maybe low 30s on a very cold day.

 

On 03/10/2024 at 11:46, Iceverge said:

My 2p worth, install big pipes and rads and continue with the boiler. It'll run more efficiently at low flow temps anyway I think .

Would agree.

On 03/10/2024 at 11:11, lookseehear said:

These things are complex and trying to navigate a path between the science and the experience of people who live with the systems is difficult without going insane!

I would step back

Decision 1 - keep or dump oil. Once made every thing else starts to fall in to place. Just jump one side or the other, don't keep jumping.

UFH downstairs - not the best solution, simple low temp radiator, with a manual TRV to manage room temp. Good for Oil or HP

Same upstairs if oil and A2A. Or fan coils for A2W. 

Thermal store for Oil, HP UVC for A2W HP.

[JamesPa]

15 hours ago, lookseehear said:

 

I like the idea of a thermal store to use excess solar for example (imagine guilt free long showers 😆) but I've been following the HeatGeek YouTube channel and they seem to believe that to have a really efficient ASHP system you need to have no buffer tank, just run weather compensation with no zoning, keeping the lowest flow temps with the highest uptime possible, so I don't want to buy into a system that if we do upgrade to ASHP in the future becomes redundant.

If all you want from your thermal store is long showers, then thats just a big domestic hot water tank, its nothing at all to do with buffer tanks.  

[Nickfromwales]

48 minutes ago, JamesPa said:

If all you want from your thermal store is long showers, then thats just a big domestic hot water tank, its nothing at all to do with buffer tanks.  

+1. Bigger tank, lower temps, less losses, no brainer. 

[ReedRichards]

16 hours ago, lookseehear said:

I've been following the HeatGeek YouTube channel and they seem to believe that to have a really efficient ASHP system you need to have no buffer tank, just run weather compensation with no zoning, keeping the lowest flow temps with the highest uptime possible

 

That's not wrong but you would have to carefully balance every radiator in the house for it to work.  Efficiency is great, it saves you money.  But you can save even more money by wearing lots of clothes and not bothering to heat your house.  So be aware that you may be sacrificing convenience for the sake of efficiency.

[Mike]

8 hours ago, JohnMo said:

Good UFH doesn't equal a warm or hot floor, poorly installed does. Well insulated, you are flowing mid to high 20s, maybe low 30s on a very cold day.

+1.

 

However without UFCH the floor will be several degrees below air temperature in mid-winter, so it's not surprising that floors feel cold. With UFCH (provided the house is air-tight and well insulated, and provided all rooms on the floor are heated to the same temperature) there is also typically 2 or 3°C less variation in air temperature between floor and ceiling, compared to using radiators.

 

All other things being equal, UFCH can therefore provide better thermal comfort but, as mentioned above, it shouldn't be the priority - airtightness, insulation and MVHR should come first.

 

Edited Saturday at 21:09 by Mike